Megohmmeter, also known as insulation resistance tester, is a kinds of instrument used to measure the high resistance with mega ohm level and to test the insulation resistance . The working principle of the megger is based on  the moving coil instrument. The principle points out that when the current carrying conductor is placed in the magnetic field, it will be affected by the mechanical force too. The magnitude and direction of the force depend on the strength and direction of the current and magnetic field.

The structure of megohmmeter:

It consists of a manual DC generator and a direct reading ohmmeter. There are two coils, PC and CC, which are fixed together at a certain angle and can rotate freely around the common axis between the poles of permanent magnet. The coil is connected to the circuit by a flexible wire (or ligament), which does not apply recovery torque to the moving system.

The current (or deflection) coil is connected in series between the  resistance R 1’s generator terminal and the test line terminal. The deflection circuit resistance R 1 limits the current and controls the range of the instrument. The pressure (or control) coil is connected in series with the compensation coil and the protection resistance (or control circuit resistance) R2 across the generator terminal.

Connect the compensation coil for a better proportional ratio. A protective ring is provided to divert leakage current above the test terminal or inside the instrument itself. A terminal "G" called a protection terminal is provided, through which the protection ring can be connected to the protection wire on the tested insulator. The test voltage generated by the generator is usually 500 or 1000 volts.

The working of megohmmeter:

The measured resistance is connected between the test terminals (L and G), and then the handle of the generator is turned stably at a uniform speed until the pointer gives a stable reading. The working principle of the megger can be fully understood from the following steps:

Step 1. When the test terminal is disconnected, the resistance to be measured is infinite. If the generator handle is rotated, the resulting voltage will send current through the potential coil, and there will be no current flowing through the current coil. Therefore, the moving system rotates in such a direction that the pointer stops at the infinite end of the scale.

Step 2. If the test terminal is short circuited and the generator is running, the generator will send a large current through the current coil, while only a small current flows through the potential coil. Therefore, the resultant torque thus generated causes the pointer to point to the zero end of the scale.

Step 3. If the unknown resistance to be tested is connected between the test terminals, a considerable amount of current will flow through both coils. The actual position occupied by the pointer depends on the ratio of current in the two coils, i.e. unknown resistance.

The meaning of zero sum infinity of megohmmeter:

When a fist shows a zero reading, this means that the unknown resistance being measured is very low. If zero value is displayed when measuring insulation resistance, it indicates insulation fault or short circuit.

When it shows an infinite reading, it means that the unknown resistance being measured has a very high value (or an open circuit).